California Classroom Science

Skywatching Activities, April and May 2011

Posted: Friday, April 1st, 2011

by Robert Victor

Several bright stars of April evenings will depart by the end of the school year. Watch them go! The April Sky Calendar and Evening Skies star map are available at www.pa.msu.edu/abrams/ABDNHAMar-AprSkies/.

Each year in the April evening sky, a large oval of bright stars, called the Winter Ellipse, or Winter Hexagon, heads toward the western horizon. Rigel, in the foot of Orion, the Hunter, will be the first of these stars to depart near the end of April. Next, in order of disappearance from mid-northern latitudes, are:

in early May: Aldebaran, eye of Taurus, the Bull

in mid-May: Sirius, the “Dog Star”; and Betelgeuse, shoulder of Orion, will actually be inside the Hexagon

in early June: Procyon, the Little Dog Star

in late June: Capella, the “Mother Goat” star; and Pollux, one of the Gemini Twins (accompanied by his fainter twin, Castor).

All but Capella and the Twins will be gone by the start of summer. These stars will appear lower each evening in spring as a result of the Earth’s revolution around the sun.

Student Activity Suggestion:

Provide each student with a star map and a checklist of these stars. Have them note on each clear evening whether or not they were able to observe each star. Ask the students to start searching for these stars within 45 minutes to an hour after sunset. It is especially important to start looking that early to catch a star approaching the western horizon during the week or so before its departure.

April

There are four additional bright stars shown on the April star map. The handle of the Big Dipper locates two of them, when you “Follow the arc to Arcturus and drive a spike to Spica.” Water leaking through the bottom of the Big Dipper’s bowl would fall on the back of Leo, the Lion, whose brightest star is Regulus. Finally, Vega, the leading star of the Summer Triangle, is just rising in the northeast.

After Vega rises and before Rigel sets, we have 11 of the 15 brightest stars (of first magnitude or brighter) ever visible from mid-northern latitudes, the greatest number possible. (Another selection of 11 will appear on our December map, after Sirius rises and before Altair sets.)

The only other bright object in the April early evening sky is Saturn. On April 3, this planet aligns with the sun and Earth in a straight line so that Saturn appears at opposition to the sun and is above the horizon all night.

Some 59 years ago, when as a 12-year-old I started noticing the positions of the planets, Saturn appeared in nearly the same place against the stars. Now, two Saturn-years later, in the spring of 2011, we can again watch Saturn appear to retrograde or move west against the same background. Note the 3rd-magnitude star Gamma in Virgo not far to Saturn’s upper right. In the second week of June, Saturn will pause just a quarter of a degree from that star.

May

The four other bright planets will form wonderful groupings best seen through binoculars very low in bright morning twilight. Drawings of the planet gathering as seen from the latitude of southern California on each morning of the month appear on the May Sky Calendar at www.pa.msu.edu/abrams/skycalendar/. Descriptions of various groupings appear in the calendar’s left margin notes.

May 11 is the date of the most compact trio of Venus, Jupiter, and Mercury, all within 2.1 degrees. Venus and Jupiter, the two brightest planets, appear closest that morning, and fainter Mars will appear within 6 degrees to Venus’ lower left, as shown in the calendar diagram for May 11.

Observers can simply follow Venus in the morning sky in the weeks and days leading up to the gathering on May 11. Sky-watchers can also determine well ahead of time exactly where the gathering will appear in relation to the local horizon features at their selected observing sitesusing the following procedure:

Observe the sun rising over the local horizon scene on Sunday, April 10, being sure to choose a place with a relatively unobstructed view in that direction.

Note the time when half the sun’s disk has risen above the distant horizon profile.

Subtract about 1 hour 44 minutes from that time, and on May 11 at this earlier time, Venus will pop into view in nearly the same place!

Here is a table of time adjustments for previewing the rising of Venus on various dates in May by viewing sunrise about a month earlier.

View to preview Look earliersunrise Venus rising by this time intervalon this date on this date to catch Venus rising
April 5 May 7 1 hr 48 min
April 10 May 11 1 hr 44 min
April 15 May 15 1 hr 40 min
April 20 May 19 1 hr 35 min
April 25 May 23 1 hr 31 min

Robert C. Victor was Staff Astronomer at Abrams Planetarium, Michigan State University. He is now retired and enjoys providing skywatching opportunities for school children in and around Palm Springs.

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4 Responses

I’m having trouble visualizing how Saturn appears to move in opposition to the sun from Earth. Do you have a simple explanation or a website with a video that demonstrates the movement of the planets and the Sun? I understand what the term means, I just don’t see why it works that way.

The apparent motion of Saturn is due more to Earth’s motion than to Saturn’s. Earth orbits much faster than Saturn, so it catches and passes it many times as both planets go around the Sun. The apparent motion of the Sun from day to day is caused entirely by Earth’s orbital motion.

A picture is probably better than words. If you have access to computer software for astronomy that shows the motions of the planets in their orbits it will help you visualize what is going on (such a display is called an orrery). A lot of such programs exist, and there are good versions for the iphone and ipad as well as for more traditional computers.

If you don’t have your own copy of such software, there are several good ones online. Here is a link I found doing a search on Google for “online orrery:”

Thank you for your inquiry about Saturn’s current retrograde motion. I hope the following modeling exercise will help students clearly understand the phenomenon of retrograde motion of planets, and that the list of upcoming cases against prominent star backgrounds will help teachers encourage students to actually get outdoors to observe and record such events!
—– Robert Victor
The outer planets, Mars, Jupiter, and Saturn, appear to retrograde, or move westward against the stars, for several weeks when the faster moving Earth overtakes them. This apparent backward motion of an outer planet can be modeled with a current example. On April 3, 2011, the Earth passed between the Sun and Saturn, and that planet was observed to appear at opposition to the Sun, 180 degrees from the Sun and visible all night.

An imaginary observer viewing from the north side of the solar system, looking down from “above” the planetary orbits, would have seen the Sun, Earth and Saturn in a straight line on April 3, with both planets moving counterclockwise in their orbits.
Each month, the Earth moves 30 degrees in its orbit, while Saturn moves just one degree. A vector from the Sun to either planet, as viewed from above the solar system, rotates counterclockwise.

The mean distance of these planets from the Sun are: Earth, 1 astronomical unit (by definition), and Saturn 9.5 astronomical units.
Next, represent the positions of Earth and Saturn in their orbits at 30-day intervals, March 4, April 3, and May 3. Remember that the positions for Sun, Earth and Saturn on April 3 lie in a straight line.

Let us first consider the change in the apparent position of Saturn against the background of distant stars, as seen from the Sun. Since Saturn moves in the same direction around the Sun all the time, it will appear to be moving to the left, or east, against the stars, as the vector from Sun to Saturn rotates counterclockwise.

But now consider the change in Saturn’s position as seen from the moving Earth. To obtain Saturn’s apparent position on March 4, draw a straight line from the Earth’s position to Saturn’s position on that date, and extend it beyond Saturn toward the very distant background of stars.

Do the same for the Earth-Saturn positions on April 3, and again for their positions on May 3.

You will notice that the lines of sight from Earth to Saturn, extended toward the distant stars, cross each other. On each successive date of March 4-April 3-May 3, Saturn appears farther west against the stars, because the direction arrow, or vector, from Earth to Saturn, is rotating clockwise as seen from above the solar system.

Retrograde motion of Saturn in 2011 occurs from January 26 until June 12, centered approximately on the date of Saturn’s opposition, April 3. During these 4.5 months, Saturn goes nearly 7 degrees west against the stars, ending near the 3rd-magnitude star Gamma in Virgo.

Have your students follow Saturn as it approaches that star this spring. See the April through June issues of the Abrams Planetarium Sky Calendar, and their accompanying evening star maps, for tips. (See the above article for web links.)
In the spring of 2012, Saturn will be about 12 degrees farther along in its orbit, and it will reach opposition on April 15. Saturn will trace out its retrograde motion near the first-magnitude star Spica.

In the school year 2011-2012, Jupiter will appear at opposition on October 28, 2011, and Mars on March 3, 2012. Around those dates each respective planet will be visible all night, appearing in the eastern sky at dusk, high in the south in the middle of the night, and low in the western sky at dawn.

In the months leading up to an opposition, a planet rises during the night and is still above the horizon at dawn. After opposition, a planet remains in the sky at dusk, gradually drifting over toward the western horizon as months pass.

Jupiter will next retrograde from Aug. 30 until Dec. 25, 2011. Near the beginning of retrograde, Jupiter will form a nearly isosceles triangle with the two prominent stars 2nd-mag. Alpha and 3rd-mag. Beta in Aries, east of (or following) the Great Square of Pegasus. In those four months, as Jupiter goes 10 degrees west, the shape of the triangle will change very noticeably.

Jupiter takes almost 12 years to complete one revolution around the Sun, so in one year it advances just over 30 degrees east against the background of the zodiac. Oppositions of Jupiter occur at intervals of about 13 months. (It takes the Earth about an extra month to catch up.) At its next opposition on December 2, 2012, Jupiter will appear near the star Aldebaran and the Hyades star cluster, which form the head of Taurus, the Bull. It will be fun to follow Jupiter as it retrogrades 10 degrees from Oct. 4, 2012 to Jan. 30, 2013. Jupiter will pass about 5 degrees north of Aldebaran three times (an event called a triple conjunction), on 2012 July 30 and December 12, and on 2013 March 18.

Mars takes under two years to revolve around the Sun, so the Earth overtakes the red planet less often than it passes the planets farther out. Mars reaches opposition every 25 to 27 months. At its next one, on March 3, 2012, Mars will appear in the constellation Leo, the Lion. In the early months of 2012, Mars will retrograde 19.5 degrees from Jan. 23 to Apr. 13, while closing from 23 degrees to about 4 degrees east of Regulus, the first-magnitude star marking the heart of Leo.

Here are a couple of activities to engage students in observing the motions of the planets: (1) Provide each student with a star map to plot the positions of planets from naked-eye observations. Each week, students should plot the position of the planet against the stars. Intervals of two weeks may suffice for plotting the positions of the slower-moving planets, Jupiter and Saturn. (2) Make a photographic record of the changing position of a planet against background stars. A series of photos can be taken to illustrate a triple conjunction of a planet with a background star, and to show the retrograde motion of a planet.

Inner planets also undergo retrograde motion! In general, a planet appears to retrograde around the date that the Earth and the other planet are on the same side of the Sun, when one planet overtakes the other. For an outer planet, this happens when the planet appears at opposition. For an inner planet (Mercury or Venus), this alignment happens when the planet is in inferior conjunction, nearly between Earth and Sun. For Venus, the next time this occurs will be on June 5, 2012, when it happens that the alignment of Sun-Venus-Earth will result in a transit of Venus across the face of the Sun. (Venus usually passes north of, or “above”, or south of, or “below”, the Sun’s disk at inferior conjunction.) Venus passes inferior conjunction about every 19.2 months, or five times in eight years, but for the next transit of Venus after June 2012, we must wait until December 2117.

Before inferior conjunction, Venus is well seen in the western evening sky. After the inferior conjunction, Venus is well seen in the eastern morning sky. In 2012, Venus undergoes retrograde motion from May 15 (when it appears within 2 degrees of 2nd-magnitude Beta Tauri, the northern horn of the Bull) until June 27 (when it appears about 3 degrees from first-magnitude Aldebaran). From early April to early July 2012, Venus will undergo triple conjunctions with Aldebaran and stars of the Hyades. But during the middle of its retrograde loop, Venus is hidden in the glare of the Sun. Nevertheless, Venus’ retrograde motion against the stars can be observed, by noting Venus’ position against the stars at nightfall about three weeks before the inferior conjunction, and in the predawn sky about three weeks after.

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